This invention concerns electron guns of the type used in television cathode ray tubes, particular emphasis being placed on the focus lens portion of such guns.
Electron guns employed in television cathode ray tubes generally comprise two basic sections: (1) an electron beam source, and (2) an electron beam focus lens for focusing the electron beam on the phosphor-bearing screen of the cathode ray tube. Most commercially employed focus lenses are of the electrostatic variety and generally are embodied as discrete, conductive, tubular elements which are arranged coaxially and which have a predetermined pattern of voltages thereon to establish the electrostatic focusing field. One commerically accepted class of such electrostatic focusing lens has been, and continues to be, the bipotential lens. The term "bipotential lens" is used herein to describe a lens, generally comprising two electrodes, which presents to electrons traveling down the lens axis from the source toward the screen target, an axial potential distribution which increases monotonically from an initial low potential near the source to a final high potential, as shown diagrammatically in FIG. 7A. The axial potential distribution of a bipotential lens of this type is said to be "monotonic" since its first derivative does not change sign.
As a class, however, the bipotential lens suffers from having undesirably poor spherical aberration characteristics and can not, in a reasonably small space such as is available in a cathode ray tube neck, provide focused beam spots sufficiently small to prevent significant loss in picture resolution, particularly at high beam current levels.
Another class of lenses, the unipotential type, has also long been known. The term "unipotential lens" is used herein to mean a lens whose axial potential distribution is substantially saddle-shaped and in which the potentials at the beginning and end of the lens are substantially equal. The axial potential distribution in such a lens decreases monotonically from an initial relatively high potential near the electron source to a relatively low potential and then increases monotonically to a final, relatively high potential. See the FIG. 7B diagram. The prefix "unit" refers to the fact that the final potential is the same as the initial potential.
Although the unipotential-type lens has achieved commerical success, it does possess an unattractive drawback related to tube internal arcing. To understand the nature of this drawback, consider that the electron source in an electron gun of the type commonly employed in cathode ray tubes comprises, along the gun axis, a cathode and two conductive grids -- a negative control grid, often described as the "G.sub.1 " electrode, and a first anode grid, commonly termed "G.sub.2 ". The G.sub.2 grid is typically excited with an applied DC voltage having a magnitude less than 1 KV (1000 volts).
The potential of the first focus lens electrode, commonly termed "G.sub.3 ", of a unipotential-type lens is, however, very large by comparison -- typically 25-30 KV. The physical separation between G.sub.2 and G.sub.3 is typically so small, considering the very high applied voltage difference therebetween, as to create an undesirably great tendency of arcing between G.sub.2 and G.sub.3. Arcing is undesirable because it is apt to damage the gun or the driving circuitry in the associated television receiver. Arcing in the electron source region is particularly undesirable since it may cause damage to the fragile cathode emission surface.
The arcing problem in a unipotential focus lens can not be overcome by simply increasing the physical separation between G.sub.2 and G.sub.3 since to do so could deteriorate the electron optical characteristics in the electron source region (cathode, G.sub.1, G.sub.2 to G.sub.3 region), or could expose the beam to extraneous external fields.
The bipotential-type lens has the important advantage over unipotential-type lenses of having a reduced susceptibility to arcing, since its initial electrode receives a much lower potential, relative to the grid G.sub.2 potential, than does the initial electrode of a unipotential-type lens. Yet another advantage of a bipotential lens is that for a given gun length it generally produces less electron optical magnification.
Still another type of lens found in the prior art (although not in the marketplace) is the periodic extended field type described for example in U.S. Pat. No. 3,702,950 and shown diagrammatically in FIG. 7C.
The focus lens provided according to the present invention takes advantage of the low aberrations produced by the extended field lens described and claimed in the referent U.S. Pat. No. 3,895,253 of J. Schwartz et al. As pointed out in that patent, it can be shown that lens aberrations depend largely on the value of the line integral of the quantity ##EQU1## where V.sub.0 is the axial potential distribution in the lens, V.sub.0 " is the second derivative of V.sub.0, and r is the beam radius. Therefore, it follows that large values of V.sub.0 " are particularly harmful in regions where the axial potential V.sub.0 is low or where beam radius is large. As in the lens of the referent patent, for the extended field lens of this invention, V.sub.0 " is substantially less over the entire lens length and is especially low in regions of low axial potential. Furthermore, the maximum values of V.sub.0 " are substantially reduced. A diagrammatical representation of the axial potential distribution of a Schwartz et al focus lens is shown in FIG. 7D.
It is noted at this time that the focusing field of the extended field lens as taught by Schwartz et al is axially continuously active. Consider the following -- a reduction in V.sub.0 " alone, especially in regions of low axial potential, might be achieved with a "composite lens" formed by placing two bipotential lenses essentially back to back separated by some predetermined axial distance. However, any reduction in V.sub.0 " would also likely be accomplished by the establishment of a drift region or inactive focusing region at the composite lens center due to the axial separation of the bipotential lenses.
The net result of the application of the afore-described Schwartz et al principles is an extended field lens in which the focusing field is spread out along the axis of the lens so that V.sub.0 varies smoothly and gradually over its entire range. The desired field characteristic can be established in the paraxial region of a very large diameter lens, however it has not been possible until the invention described in the referent copending Schwartz et al application to achieve the desired field characteristic in a lens having a small diameter. It has been found that by keeping the quantity V.sub.0 " as small as possible in regions where V.sub.0 is small or where the beam diameter is large, the necessary focusing power can be achieved while suppressing the total spherical aberration produced.
It has been concluded that if high picture brightness (implying relatively high beam currents) and high resolution (implying relatively small focused beam spot size) are simultaneously desired, one must look to something other than the standard bipotential or unipotential lenses. These objectives are met by the present invention. The invention will be described at length below; however, in order to quickly place the invention in the context of the FIGS. 7A-7D diagrams, reference may be had to FIG. 7E which reveals the very novel axial potential distribution of an exemplary focus lens constructed according to the teachings of this invention.